The 52-year-old victim told police that without warning, the attacker picked up a large bream from the stall and hit her across the face before running out of the store.

She told officers that she believed the slap was being filmed on a phone by a man who was stood nearby.

He also ran from the Hyndburn Road shop when challenged.

Hyndburn MP Graham Jones condemned the attack.

He said: “If this was done as a joke or a dare, then there is a big issue with some of our young people.

“Abusing hardworking people in a supermarket does not strike me as fun.

“These people work all day just to make ends meet and they do not need customers coming in and being abusive or playing pranks or tricks.

“We have got to ask the question why somebody would find this funny or entertaining and where they see their futures doing things like this.”

Hyndburn borough councillor Peter Britcliffe said he was concerned the incident could be part of a social network craze like the neknominate drinking game.

He said: “This is a shocking thing to happen and if this is some sort of prank, it is stupid.

“It would humiliate the person who had been hit, but it is also quite a dangerous thing to do.

“Some of these silly pranks that are going round at the moment are stupid and I hope this has not been inspired by social media.”

Fish slapping was first seen as part of a Monty Python sketch featuring John Cleese and Michael Palin wearing safari outfits.

In the sketch Palin uses two small fish to hit Cleese several times before Cleese strikes him with a much larger one and knocks him into a quay.

An investigation into the Accrington assault, which happened around 3pm on February 4, has been launched and police have released CCTV images of two people they believe could have vital information.

PC Graham Hartley appealed for anybody with information or who might have seen the man and woman to come forward.

The woman was described as being Irish and aged between 15 and 18 with blonde, shoulder-length, untidy hair.

She was wearing dark jeans and a blue coat with white cuffs, white down the front and a red collar with ‘I love PB’ on the left breast pocket.

The man is thought to have been around the same age, was 5ft 8ins tall, wearing a dark coat and dark grey trousers and boots.

PC Hartley said: “This behaviour is completely unacceptable and I would appeal to anybody that witnessed this incident or recognises either the man or the woman pictured in the CCTV images to come forward and contact me.”

A spokesman for Asda said: “We absolutely do not condone any form of assault on our colleagues.

“We are aware of the incident and assisting the police in their investigation.

“We urge anyone who has any information to get in touch with their local constabulary.”

Anybody with information should call police on 101 or Crimestoppers anonymously on 0800 555111 .